Example of a wire-disconnection detecting device for such a wire cutting type electrical discharge machining system, are disclosed in the TOKU-KAI-SHO 53 (1978)-68496 (a laying-open publication of a Japanese patent application). In FIG. 3 of the publication one device is illustrated which includes a detecting power supply separate from a machining power supply, and a detector to sense the disconnection of the wire electrode. The detecting power supply gives a small amount of current to the wire electrode through a pair of power-supply contacts disposed above and below a work table for supporting the workpiece, and the detector detects a variation of the current between the two power supply contacts upon disconnection of the wire electrode. FIG. 4 in the same publication shows another device wherein a current transformer is disposed, in place of the detector of FIG. 3, adjacent to the wire at a portion located between the pair of power supply contacts. FIG. 4 also shows another device wherein the current transformer is also provided, but a machining power source is connected to both the power-supply contacts. In those devices, wire-disconnection is detected by sensing the absence of a current flow through the wire electrode.
If the device of FIG. 3 is adapted such that the machining power source is connected to both of the power-supply contacts, the small amount of current flows through the power line of the machining power supply circuit in the event of disconnection of the wire electrode, with a disadvantageous result of preventing the wire-disconnection detection by the detector. The device of FIG. 4 is problematic, because the output signal, which is generated upon detection by the current transformer of a variation of the current through the wire electrode, is weak. This means that the output of the current transformer must be amplified by a costly means such as an amplifier. It is true that when the machining power source is connected to the upper and lower power-supply contacts in addition to the installation of the current transformer for current detecting, strong output signal can be obtained, and such a disadvantage as observed in the device shown in FIG. 3 can be prevented, i.e., a wire-disconnection in the device, in which the machining power source is connected to the upper and lower power-supply contacts, will not change the closed state of the detector circuit, which naturally prevents detection of the wire-disconnection. In other words, the above arrangement is of some merit per se, but the application of this type of detecting arrangement is limited to a machining system wherein the machining power supply is connected to the upper and lower power-supply contacts.